Laminated billets are used in thermoforming and solid phase forming. The typical method for forming the laminated billet involves extruding a plurality of molten sheets, laminating the sheets together by passing them between rollers, and thereafter cutting or stamping the billets from the laminated sheets. This leaves a laminated web and unless this web can be re-used effectively, this process of producing a laminated billet is made more expensive. Attempts are made to reintroduce the plastic from the web into the billet by either making a sheet from the mixed plastics of the web, or by mixing the plastics from the web into one of the layers. Neither one of these solutions is satisfactory because the sheets of mixed plastics or the blends of the mixed plastics with virgin material do not give the optimum properties nor do they process well.
It is possible to cool each of the sheets exiting the extruder below their softening point, cut individual billets from each sheet, then laminate the individual billets; but this requires handling a plurality of billets which, given the complexity of forming a laminate from a plurality of individual billets, has proven to be a costly manual handling problem.
Of course it should be well recognized that radio frequency radiation (about 01 to about 300 MHz) can be used to heat the billet layers. As used herein, dielectrically or radio frequency heatable thermoplastics are those having a loss index greater than about 0.8, more preferably greater than about 0.9 and most preferably greater than about 1.0 at the frequency of irradiation and these thermoplastics can be made to heat and melt when subject to an alternating radio frequency field. Furthermore, the nature of radio frequency heating allows one to be relatively selective about the geometry of heating. That is, one can localize the heating of a plastic to be essentially that volume of lossy material between the radio frequency electrodes.
If, for example, a laminate is to be made of a plurality of thermoplastic layers, at least one of which has a loss index equal to or greater than 0.8, and the thickness of the laminate, that distance perpendicular to the layers, is small compared with the distance parallel to the layers, one can melt the heatable layer with radio frequency radiation, and by conduction from the heatable layer cause any adjacent non-radio frequency heatable layers to melt. Assuming the adjacent layers are compatible, the layers will, with a small amount of pressure, form a laminate.
A process therefore need be developed that eliminates the necessity of handling individual billets or a stack of loose individual billets, and that leads to the direct formation of billets. The process should also minimize the energy loss and maximize the polymer utilization when manufacturing laminated billets from extruded sheets.